Depending on map layout rockets can be used.. It has to be really open though, ie. no center peice... and with the center peice being the height of foundry mlg maps, its rare. Not to mention the map would most likely play bad.
Yeah, I wouldn't encourage you to say things like "MLG has deemed Foundry too small for this weapon" for snipers and rockets. I believe the real reason they were not used goes back to its gameplay. While Foundry is small, it takes similar times to traverse on maps like Guardian or Lockout, and we see snipers there. Guardian and Lockout are multi-leveled maps, with less ways to get from one level to the other than most Foundry maps (due to budget and item restrictions, Bungie will NOT let us use those glitches on any of their playlists). This slows down the gameplay tremendously, and allows more isolated battles to take place. In Amp, Onslaught, Hype, Lockdown, etc the maps require team BRing, and once your low on shields, it's unlikely that you'll be able to regenerate them. So theoretically, if a map was more enclosed, slowing the gameplay down, then power weapons and powerups would be more viable to use. Just something to consider. I'm not encouraging anyone or discouraging them.